packardv8

Surprised no one has mentioned using a battery maintainer/trickle charger. These can prolong battery life, especially if it's not a daily driver. These can also be used to supply power while the battery is being replaced, preventing loss of information/throwing codes.

jack vines

Martigary

I use a CTEK battery charger to "feed" my vehicles batteries when they are not used for a week or so.
I have a CTEK MXS - 7.0 from a local auto parts store which was 1/2 the price of the Audi OEM charger (420 093 050 E), that happens to be a rebadged version of the CTEK one.

ronsc1985

One can easily tell the battery condition by looking up the percent charge vs no load voltage on many websites. They also list the correction factor for temperature for the various battery types. In order to do this you need a digital voltmeter capable of 10 millivolt accuracy . Just park the car after running long enough to charge the battery, let it sit overnight and then read the battery voltage. Use the charts to decide your percent charge and then decide how lucky you feel if the percent is below 50%. When deciding how lucky you feel remember the car will always not start at a location and time which maximizes your inconvenience.